1. Field of the Invention
The present invention relates to a solid electrolytic capacitor including an anode element made of a sintered body of a valve-action metal and an anode lead member projecting from the anode element, and to a manufacturing method therefor.
2. Description of Related Art
A solid electrolytic capacitor having a structure shown in FIG. 9 has been conventionally known. The solid electrolytic capacitor includes an anode element 31 made of a sintered body of powder of a valve-action metal (tantalum, niobium, titanium, aluminum, etc.). Sequentially formed on a surface of the anode element 31 are a dielectric coating 33 made by oxidizing the anode element surface, a solid electrolyte layer 34 made of a conductive inorganic material such as manganese dioxide or a conductive organic material such as TCNQ complex salt, a conductive polymer, etc., and a cathode lead layer 35 made of a carbon layer, silver layer, etc. An anode lead member 26 made of a conductive metal projects from the anode element 31 in one direction. As shown in FIG. 10, the anode lead member 26 is formed in the form of a flat quadrangular prism with a rectangular cross-sectional shape, and has a base end thereof embedded in the anode element 31.
As shown in FIG. 9, an anode lead frame 40 is connected to the anode lead member 26, while a cathode lead frame 41 is connected to the cathode lead layer 35 through a conductive adhesive 36. A periphery of the anode element 31 is coated and sealed by an enclosure resin layer 37 made of epoxy resin or the like. The anode lead frame 40 and cathode lead frame 41 are partly exposed from the enclosure resin layer 37.
As shown in FIG. 10, the anode lead member 26 formed in the form of a flat quadrangular prism with a rectangular cross-sectional shape can provide a larger joint area between the anode lead member 26 and the powder of the valve-action metal of the anode element 31 than a cylindrical anode lead member with a circular cross-sectional shape. Therefore, a resistance is reduced at a joint point between the anode lead member 26 and the valve-action metal powder of the anode element 31 to lower an equivalent series resistance (ESR). Furthermore, a large current can flow through the solid electrolytic capacitor by increasing the joint area between the anode lead member 26 and the valve-action metal powder of the anode element 31 (see JP 3535014, B).
The anode element 31 having the anode lead member 26 projected therefrom as shown in FIG. 10 is prepared by vacuum-sintering a molding of the valve-action metal powder having the base end of the anode lead member 26 embedded therein. However, the anode element 31 having the anode lead member 26 projected therefrom as shown in FIG. 10 has a problem of, as shown in FIG. 11, occurrence of a crack 30 from a boundary between the anode lead member 26 and anode element 31, in particular, from corners of the anode lead member 26 in a face 32 of the anode element 31 having the anode lead member 26 projected therefrom after the preparation of the anode element 31 by vacuum-sintering the molding of the valve-action metal powder.
Even if the anode element 31 and anode lead member 26 are of the same material, the anode element 31 can have a smaller density than the anode lead member 26, such that the anode element 31 can have a larger linear expansion coefficient than the anode lead member 26 due to the difference in density between the anode element 31 and anode lead member 26. This causes the anode element 31 to contract more greatly than the anode lead member 26 when the anode element 31 is cooled after the molding of the valve-action metal powder is vacuum-sintering to prepare the anode element 31. The thermal stress concentrates especially on the corners of the anode lead member 26 of the anode element 31. This is probably the cause of the occurrence of the crack 30.
The occurrence of the above-described crack 30 at the anode element 31 increases a leak current at the anode element 31, and therefore degrades the performance of the solid electrolytic capacitor to lower the yield.